The precise tracking of distance variations between two satellites in low Earth orbit can provide key data for the understanding of the Earth’s system, specifically on seasonal and sub-seasonal water cycles and their impact on water levels. Measured distance variations, caused by local variations in gravitational field, serve as inputs to complex gravity models with which the movement of water on the globe can be identified. Satellite missions GOCE (2009–2013) and GRACE (2002–2017) delivered a significant improvement to our understanding of spatial and temporal gravity variations. Since 2018, GRACE Follow-On has been providing data continuity and features for the first time through the use of a laser interferometer as the technology demonstrator, in addition to a microwave ranging system as the main instrument. The laser interferometer provides an orders-of-magnitude lower measurement noise, and thereby could enable a significant improvement in the measurement of geoids if combined with suitable improvements in auxiliary instrumentation and Earth system modelling. In order to exploit the improved ranging performance, the ESA is investigating the design of a ‘Next Generation Gravity Mission’, consisting of two pairs of satellites with laser interferometers, improved accelerometers and improved platform performance. In this paper, we present the current design of the laser interferometer developed by us, the development status of the individual instrument units and the options available.

The applied principle for measuring the distance variation Δd of two satellites, flying approximately 100 km apart from each other at the same orbit with nanometer resolution, is heterodyne laser interferometry, similar to the one foreseen for LISA. Laser beams are exchanged between the satellites and small distance variations are measured. The distance variation is caused by residual air drag and differences in the local gravity forces. The variation the optical path travelled by the laser beams can be detected as a variation of the phase angle of the interferometer beat signals. The achievable measurement resolution is a small fraction of the laser wavelength. To derive the gravity component of the distance variation, the air drag component is measured with an accelerometer at the center of mass of each satellite and subtracted from the interferometer ranging signal.

The measurement of the static and temporal variation of Earth’s gravity field yields important information on water storage, seasonal and sub-seasonal water cycles, their impact on water levels and delivers key data to Earth’s climate models. The satellite missions GOCE (ESA), GRACE (US-DE) and just recently GRACE Follow-On (US-GER) resulted in a significant improvement on our understanding of the system Earth. ESA is investigating the concept of a ‘Next Generation Gravity Mission’ (NGGM) as a Mission of Opportunity for ESA-NASA cooperation in view of their Mass Change and Geoscience International Constellation (MAGIC) [9]: it consists of two pairs of satellites embarking a heterodyne laser interferometer for inter-satellite ranging, the so called Laser Tracking Instrument (LTI), to further improve the measurement accuracy of the time-variable Earth gravity field.

The Gravity Recovery And Climate Experiment (GRACE) Follow-On mission was launched on May 22, 2018, to continue monitoring changes in the gravity field of the Earth by measuring distance variations between two spacecraft that fly 200 km apart in a low-Earth polar orbit. The laser ranging interferometer (LRI), a technology demonstrator onboard GRACE Follow-On, is the first of its kind to perform interspacecraft ranging measurements and has shown noise levels of 1 nm/ Hz at 100 mHz and 200 pm/ Hz at 5 Hz. Its development was shared between parties in Germany and the United States. A key optical component for the LRI’s success is the triple mirror assembly (TMA), which acts as a corner-cube retroreflector and enables the laser link between the two spacecraft. This paper presents the TMA design and characterization from the unit level to measurements in orbit. The in-orbit measurements furthermore provide the far-field intensity distribution of the Gaussian beams exchanged between the spacecraft after traveling 200 km. We address lessons learned that have influenced the design of the next generations of the LRI.